1. Field of the Invention
This invention relates to an optical wave guide array adapted for use in optical recording apparatus.
2. Description of the Related Art
In recent years, optical recording apparatus using an optical wave guide array wherein a great number of optical wave guides are arranged in lines, have been employed instead of optical recording apparatus using f.theta. lenses.
Such an optical wave guide array has been heretofore fabricated according to an injection molding technique or a selective polymerization technique.
In the injection molding technique, a resin having a low refractive index is injected into a mold to form a substrate also serving as a clad substrate wherein a multitude of grooves at equal pitches are formed. Subsequently, a part of the mold is removed, after which a core material-filling mold is placed so that cavities are formed at the positions where the grooves have been formed in the clad substrate. A core material having a high refractive index is packed in the respective cavities to form a core portion. Thereafter, a covering material serving as a clad portion is used to cover the clad substrate and the core portion to provide an optical wave guide array. In the injection molding techniques described above, polycarbonates, polymethyl methacrylate and polystyrene have been in use as optically transparent, injection-moldable materials. For instance, when the optical wave guide array is made from a combination of polycarbonate and polymethyl methacrylate, the polymethyl methacrylate having a relatively small refractive index is used as the clad substrate and the polycarbonate having a relatively great refractive index is employed as the core portion.
On the other hand, in the selective polymerization technique, a UV-curable resin is irradiated with an ultraviolet ray through a mask to make a difference in refractive index between the UV-irradiated portion and the mask-shielded substrate, thereby forming a clad portion and a core substrate, followed by covering the clad portion and the core portion with a material having a low refractive index and serving also as a clad portion to obtain an optical wave guide array.
However, in the injection molding technique, when the core material is filled in the grooves of the clad (substrate), the cladding material of the substrate provided in the mold has to be heated to a temperature necessary for the filling of the core material. This invites thermal expansion of the cladding material, sometimes leading to deformation of the grooves. For example, when the core material used is a polycarbonate, the mold temperature at which the polycarbonate is injected is as high as approximately 100.degree. C. If a resin such as polymethyl methacrylate or polystyrene is used as a cladding material for the substrate, the cladding material is softened and is likely to undergo thermal deformation of the grooves. The injection pressure for filling, the core material in the grooves of the clad substrate is required to be approximately 500 kg/cm.sup.2 even if a resin having good fluidity is used. Thus, the grooves of the clad substrate (which has been thermally expanded or softened) are further subjected to the injection pressure of the core material, making it difficult to fabricate an optical wave guide array with a precise dimensional accuracy owing to the deformation of the grooves. Further, where the core portion has a size on the order of micrometers, fabrication of the clad substrate by the injection molding technique is possiible, with the problem, that an extreme difficulty is involved in filling the core material in the grooves of the clad substrate. Moreover, if the groove size of the clad substrate is small, there arises a problem on the releasability between the mold for forming the clad substrate and the grooves of the clad substrate, with an attendant problem that a part of the clad substrate may be broken.
On the other hand, in the selective polymerization technique, since the core substrate and the clad portion are simultaneously formed according to the polymerization reaction caused by UV irradiation, the interface between the clad substrate and the core portion becomes indefinite, with the problem that the transmission efficiency of a light beam is lowered. After the UV irradiation, the clad substrate and the core portion have to be bonded with a film and a reinforcing material, so that it has been difficult to attain a good registration accuracy between the clad substrate and the core portion. For solving these problems, there is known a method wherein a substrate is fabricated by injection molding and a core material is filled in the grooves of the substrate at normal temperatures and pressures. In this case, the core material is required to be a liquid wherein volatile components are contained in small amounts and which has a low viscosity. In general, UV-curable resins are most suitable for this purpose.
Ordinarily employed UV curable resins are acrylic resins such as of acrylic esters, urethane acrylates, epoxy acrylates and the like. However, a problem is involved in that polymethyl methacrylate ordinarily used as the substrate is attacked by the UV-curable resins. In order to solve this problem, it may occur that fluorine-containing resins which are resistant to acrylic resins are used as the cladding material. The fluorine-containing resins are expensive, with an attendant problem that the fabrication costs of the optical wave guide array become high.